The integration of digital flat panel detectors into the C-arm of angiography systems allows CT-like imaging in the angiography laboratory. In the case of these C-arm computed tomography scans it may happen that the object to be imaged extends beyond the scan field of view, with the result that data capture produces truncated projection data.
CT-like imaging means in particular generating volume images with low-contrast resolution which allows different soft tissues to be differentiated. An example of this is the product DynaCT of Siemens AG, Medical Solutions, based on the AXIOM Artis systems as described, for example, in AXIOM Artis FD Systems—DynaCT—A Breakthrough in Interventional 3D Imaging, by Patrick Kurp, Reprint from Medical Solutions, January 2005, pages 46 to 51, or as disclosed in US 2006/0120507 A1.
FIG. 1 shows an x-ray diagnostic device which has a C-arm 2 rotatably mounted on a stand in the form of a 6-axis industrial robot or articulated arm robot 1, at the ends of which C-arm are mounted an x-ray source, e.g. an x-ray emitter 3, and an x-ray detector 4 as an image capture unit. The above-mentioned CT-like imaging can be performed using x-ray diagnostic equipment of this kind, for example.
By means of the articulated arm robot 1 disclosed in U.S. Pat. No. 7,500,784 B2, for example, which preferably has six axes of rotation and therefore six degree of freedom, the C-arm 2 can be placed in any spatial position by rotating it, for example, about a rotation center between the x-ray emitter 3 and the x-ray detector 4. The inventive x-ray system 1 to 4 is in particular rotatable about centers of rotation and axes of rotation in the C-arm plane of the x-ray detector 4, preferably about the center of the x-ray detector 4 and about axes of rotation intersecting the center of the x-ray detector 4.
The known articulated arm robot 1 has a base frame which is e.g. fixed to a base. Rotatably mounted thereon about a first axis of rotation is a carousel. Pivotally mounted on the carousel about a second axis of rotation is a rocker arm on which a robotic arm is rotatably mounted about a third axis of rotation. Rotatably mounted to the end of the robot arm about a fourth axis of rotation is a robotic hand. The robotic hand has a fixing element for the C-arm 2 which is pivotable about a fifth axis of rotation and rotatable about a sixth axis of rotation running perpendicular thereto.
The implementation of the x-ray diagnostic device is not dependent on the industrial robot. Normal C-arm machines can also be used.
The x-ray detector 4 can be a rectangular or square flat semiconductor detector made of amorphous silicon (a-Si), referred to as a digital flat panel detector.
Located in the beam path of the x-ray emitter 3 is a laterally displaceable and height-adjustable patient positioning table 5 for use in angiography for scanning e.g. the heart of a patient 6 to be examined as an examination subject. A system control unit 7 comprising an imaging system 8 which receives and processes the image signals of the x-ray detector 4 is connected to the x-ray diagnostic device. The x-ray images can then be viewed on a monitor 9.
The x-ray emitter 3 emits a radiation beam 10 originating from a beam focus of its x-ray source which is incident on the x-ray detector 4. If 3D datasets are to be produced in accordance with the above mentioned DynaCT method, the rotatably mounted C-arm 2 with x-ray emitter 3 and x-ray detector 4 is rotated such that, as shown schematically in FIG. 2 in plan view onto the axis of rotation, the x-ray emitter 3 here represented pictorially by its beam focus and the x-ray detector 4 move in an orbit 12 around an examination subject 11 located in the beam path of the radiation beam 10 of the x-ray emitter 3. The orbit 12 can be completely or partially traveled to produce a 3D dataset.
Data acquisition takes place during rotation of the C-arm around the examination subject 11, the focus of the x-ray emitter 3 moving on a predefined trajectory, e.g. on a graduated circle, the oppositely disposed x-ray detector 4 recording two-dimensional projection images. The latter are used as two-dimensional projections from which the volume dataset is computed using known cone beam CT algorithms such as the so-called Feldkamp algorithm, as described by Feldkamp et al. in the article “Practical cone-beam algorithm”, Journal of the Optical Society of America, Vol. 1, No. 6, Jun. 1984, pages 612 to 619.
According to the DynaCT method, the C-arm 2 with x-ray emitter 3 and x-ray detector 4 preferably moves through an angular range of at least 180°, e.g. 180° plus fan angle, and captures projection images from different projections in rapid succession. Reconstruction may only take place from a portion of this captured data.
The examination subject 11 can be, for example, an animal or human body or even a phantom.
The x-ray emitter 3 and the x-ray detector 4 each move around the subject 11 such that the x-ray emitter 3 and the x-ray detector 4 are on diametrically opposite sides of the subject 11.
In normal radiography or fluoroscopy by means of an x-ray diagnostic device of this kind, the 2D medical data of the x-ray detector 4 may be buffered in the imaging system 8 and subsequently displayed on the monitor 9.
As the digital flat panel detectors currently used as x-ray detectors 4 have limited field sizes, e.g. 30×40 cm2 in the case of AXIOM Artis, for larger organ areas such as in the thorax and abdomen it is not possible to map the anatomy in question completely onto the x-ray detector 4. This means that the two-dimensional projection is unable to capture edges of the organ area.
However, with cut-off, i.e. truncated, projections of said kind, artifacts are produced in the volume images in the reconstruction, said artifacts in some cases masking details of the anatomy captured. For more accurate reconstruction, therefore, a truncation correction is applied to the projections. To achieve this, various methods are known from the relevant literature which basically consist of extrapolating the truncated projections at the edge to zero on the basis of a model, see e.g. Bernd Ohnesorge et al., “Efficient correction for CT image artifacts caused by objects extending outside the scan field of view”, Med Phys, Vol. 1, pages 39 to 46, 2000.